Dissertations, Theses, and Capstone Projects

Date of Degree

9-2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Physics

Advisor

Swapan K. Gayen

Advisor

Maria C. Tamargo

Committee Members

Carlos Meriles

Min Xu

Pradip Bhowmik

Subject Categories

Atomic, Molecular and Optical Physics | Laboratory and Basic Science Research | Optics | Polymer and Organic Materials | Quantum Physics | Semiconductor and Optical Materials

Keywords

ZnCdSe/ZnCdMgSe multi-quantum-well heterostructures, Molecular-beam-epitaxy, Vertical-external-cavity-surface-emitting lasers (VECSELs), Time-resolved fluorescence and pump-probe spectroscopy, Pyrylium salts for dye lasers, Nonlinear-optics: Two-photon excitation and z-scan-technique

Abstract

Materials with robust optical spectroscopic and nonlinear optical properties are of avid interest for a variety of applications including lasers, light-emitting diodes, photovoltaic devices, solar cells, contrast agents for one-photon and multi-photon imaging, optical limiters, to name a few. The research presented in this thesis explores two types of materials – two II-VI semiconductor heterostructures, and several pyrylium (Py) salts for their potential photonic applications. More specifically,  the objectives of the research presented in this thesis are to:

(a)   Study the structural, optical spectroscopic, and optical gain characteristics of epitaxially grown II-VI semiconductor ZnCdMgSe/ZnCdSe multiple quantum well (MQW) heterostructures for potential applications as active materials for surface emitting semiconductor disk lasers (SDL) operational in the 480 – 630 nm spectral range;

(b)   Explore the potential of several Py-salts as active materials for dye laser and contrast agents for biological and biomedical imaging applications; and

(c)   Study the nonlinear optical properties of bispyrylium tosylate, a pyrylium salt that is structurally different from the salts with lasing potential.

We have studied the structural, optical spectroscopic and gain characteristics of ZnCdSe/ZnCdMgSe heterostructures grown lattice matched on InP substrates using the molecular beam epitaxy (MBE) technique. X-ray diffraction (XRD) scans revealed excellent crystalline quality of the samples. The photoluminescence (PL) spectra of the samples span 540 – 580 nm spectral range with peak at 560 nm and have a room-temperature lifetime of ~ 1 ns. Under more intense ultrashort, pulsed excitation, time-resolved PL measurement with a streak camera revealed  stimulated emission (SE) centered at 568 nm, spectral narrowing, and lifetime shortening to ~ 100 ps, which are indicative of stimulated emission and potential for laser action. Variable stripe-length (VSL) gain measurements yield modal gain coefficients up to ~478 ±10 cm-1.

Time-resolved ultrafast excite-and-probe measurements revealed high probe amplification with peak gain coefficient of ~104 cm-1 at a ~100 ps pump-to-probe delay and some salient features of excited-state carrier dynamics and ultrafast quantum confined Stark effect.  We present theoretical calculations of the band-to-band transitions in the quantum wells to develop a detailed understanding of the various features observed in the optical spectroscopic and gain characterization measurements. Transitions obtained with modulation spectroscopic techniques such as contactless electro reflectance (CER) and time-resolved pump-probe photoreflectance (PR) corroborate the calculated band-to-band transitions.

Pyrylium salts are cationic molecules with high solubility in most organic solvents. The basic structure of these salts consists of a six-membered hetero-cyclic aromatic ring with a trivalent oxygen atom (a donor) that can bond with negatively charged counter ions to provide salts that fluoresce over a broad spectral range of 400 – 650 nm .

We investigated the optical spectroscopic properties of 22 Py-salts with different counter ions broadly classified as Py-tosylates, Py-triflimides, Py-chloride, Py-tetrachloroferrates, and poly-pyridinium salts grown using organic synthesis methods by our collaborators. Solutions of these Py-salts in organic solvents such as acetonitrile, methanol, ethanol, acetone, dichloromethane, toluene, chloroform, and occasionally water exhibit strong absorption in the 270 - 470 nm spectral range and emission in the 450 – 560 nm range, and the fluorescence quantum yields ranging between 0.05 – 0.87. Time-resolved fluorescence measurements using ultrashort pulsed excitation into the absorption bands and streak camera detection of fluorescence lifetime demonstrated lifetime shortening (from a few ns to ~100 ps) and spectral narrowing (from FWHM of 60 nm to 20 nm) at high pump intensities. Free-running laser action was observed with minimal feedback  from the cuvette side walls.

Most of these Py-salt solutions show strong two-photon excitation (TPE) induced fluorescence with TPE cross section in the 30 – 800 GM range which is indicative of their potential application in two-photon microscopy of biological cells and tissues. Py-salts with high quantum yields tend to be the ones with the larger TPE cross sections.

While most of the pyrylium salts that we investigated are strong light absorbers and emitters, the salt bispyrylium tosylate exhibits a low fluorescence quantum yield, but a strong third-order optical nonlinearity in organic solvents. Large third–order optical nonlinearity (γ) on the order 10-14 cm2/W with a negative sign has been observed for bispyrylium tosylates in acetonitrile using the z-scan technique, a phase sensitive detection method based on the principles of on-axis spatial beam distortion. The measurements also provided a negative third-order nonlinearity of ~10-15 cm2/W for pure acetonitrile.

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